02-07-2013, 02:18 PM
IBOC AM & FM – IMPLEMENTATION ISSUES
[attachment=56319]
ABSTRACT
Digital Radio Broadcasting is off to a fast start in the USA with iBiquity IBOC (In-Band,On-Channel) as the FCC has recently approved both the AM & FM IBOC systems for use.
What benefits might this technology offer the rest of the world? How does IBOC solve the problems that other Digital Radio systems have faced? How is IBOC implemented, and what technical performance does it offer? What are the commercial realities of IBOC–why might this system succeed where others have failed?
It appears to be a certainty that digital broadcasting is in the future for most of today’s broadcasters,
and that the IBOC technology will be the method utilized in the United States and in much of the
Americas. To date, over 100 radio stations in the US have already purchased IBOC transmission
systems, and many receivers have been introduced for it.
IBOC: WHAT IS IT ?
iBiquity IBOC is a method of broadcasting digital radio broadcasting signals on the same channel,
and at the same time as the conventional AM or FM signal. iBiquity has developed both AM and
FM solutions in response to the need in the US for a digital system that didn’t require additional
frequency bands which were not available.
FM IBOC is a OFDM (orthogonal frequency division multiplex) system which creates a set of
sidebands each side of the normal FM signal. The combined FM and IBOC signal fits in the same
spectral mask as is specified for conventional FM. The system includes multiple steps towards
eventual full utilization of the spectrum by the digital signal.
Although most people are aware of the IBOC technology, there are many misconceptions regarding
its implementation, especially among non-technical people. Let’s address a few of these.
HIGH LEVEL COMBINING
Separate exciters and power amplifiers are used to generate the analog FM and IBOC signals.
Typically the analog section will consist of the station’s existing transmission equipment. A new
IBOC exciter and power amplifier are added, as well as a final combiner, which is used to combine
the output of both transmitters at the entrance to the antenna system. The power output required for the digital transmitter needs to be determined based on the system
requirements. Begin with the IBOC standard that the transmitted RF power for the digital signal
should be 22 dB less than for the analog signal.
LOW LEVEL COMBINING
In the second option, the analog and digital are combined at the output of the exciters, and the
resulting complex signal is amplified by a common RF amplifier. This amplifier must be specially
designed to pass the IBOC signal – much greater linearity is required than is currently needed for
today’s FM broadcasting.
This means a low efficiency broadband solid-state amplifier. Tube transmitters, with their class C
amplifiers and tuned cavities, and solid state transmitters with tuned power amplifiers are not
capable of the linearity necessary for IBOC transmission.
If you have a solid-state transmitter, it may be possible to adapt it to pass the IBOC digital signal.
But it must be a wide-band design (without tuned PA amplifiers). The amplifiers will need to be
converted from Class C operation to Class A or AB. Close cooperation with the manufacturer will
be required to achieve this conversion. Once converted, the transmitter will probably lose between
30 and 60% of its output power capacity.
THE COSTS OF CONVERSION
The total cost of converting to IBOC transmission will depend on several factors, among these:
• What is the station’s power?
• Will high or low level combining be used? (FM)
• Are there any antenna system issues (AM)
• Is the existing equipment up to date? Does it meet the IBOC requirements, or are manufacturer-
supported modifications available?
• Are digital studios and STL’s already in place?
[attachment=56319]
ABSTRACT
Digital Radio Broadcasting is off to a fast start in the USA with iBiquity IBOC (In-Band,On-Channel) as the FCC has recently approved both the AM & FM IBOC systems for use.
What benefits might this technology offer the rest of the world? How does IBOC solve the problems that other Digital Radio systems have faced? How is IBOC implemented, and what technical performance does it offer? What are the commercial realities of IBOC–why might this system succeed where others have failed?
It appears to be a certainty that digital broadcasting is in the future for most of today’s broadcasters,
and that the IBOC technology will be the method utilized in the United States and in much of the
Americas. To date, over 100 radio stations in the US have already purchased IBOC transmission
systems, and many receivers have been introduced for it.
IBOC: WHAT IS IT ?
iBiquity IBOC is a method of broadcasting digital radio broadcasting signals on the same channel,
and at the same time as the conventional AM or FM signal. iBiquity has developed both AM and
FM solutions in response to the need in the US for a digital system that didn’t require additional
frequency bands which were not available.
FM IBOC is a OFDM (orthogonal frequency division multiplex) system which creates a set of
sidebands each side of the normal FM signal. The combined FM and IBOC signal fits in the same
spectral mask as is specified for conventional FM. The system includes multiple steps towards
eventual full utilization of the spectrum by the digital signal.
Although most people are aware of the IBOC technology, there are many misconceptions regarding
its implementation, especially among non-technical people. Let’s address a few of these.
HIGH LEVEL COMBINING
Separate exciters and power amplifiers are used to generate the analog FM and IBOC signals.
Typically the analog section will consist of the station’s existing transmission equipment. A new
IBOC exciter and power amplifier are added, as well as a final combiner, which is used to combine
the output of both transmitters at the entrance to the antenna system. The power output required for the digital transmitter needs to be determined based on the system
requirements. Begin with the IBOC standard that the transmitted RF power for the digital signal
should be 22 dB less than for the analog signal.
LOW LEVEL COMBINING
In the second option, the analog and digital are combined at the output of the exciters, and the
resulting complex signal is amplified by a common RF amplifier. This amplifier must be specially
designed to pass the IBOC signal – much greater linearity is required than is currently needed for
today’s FM broadcasting.
This means a low efficiency broadband solid-state amplifier. Tube transmitters, with their class C
amplifiers and tuned cavities, and solid state transmitters with tuned power amplifiers are not
capable of the linearity necessary for IBOC transmission.
If you have a solid-state transmitter, it may be possible to adapt it to pass the IBOC digital signal.
But it must be a wide-band design (without tuned PA amplifiers). The amplifiers will need to be
converted from Class C operation to Class A or AB. Close cooperation with the manufacturer will
be required to achieve this conversion. Once converted, the transmitter will probably lose between
30 and 60% of its output power capacity.
THE COSTS OF CONVERSION
The total cost of converting to IBOC transmission will depend on several factors, among these:
• What is the station’s power?
• Will high or low level combining be used? (FM)
• Are there any antenna system issues (AM)
• Is the existing equipment up to date? Does it meet the IBOC requirements, or are manufacturer-
supported modifications available?
• Are digital studios and STL’s already in place?